This invention involves the superplastic shaping and diffusion bonding of metal parts and, in particular, the shaping and bonding of hollow parts in a frangible die.
Superplastic forming and diffusion bonding have been found to have a number of advantages in the manufacture of parts, particularly for high strength, light weight, aerospace applications. A number of high performance alloys, such as titanium and aluminum alloys, exhibit superplasticity; that is, the capability of developing unusually high tensile elongation with little tendency toward local necking during deformation. Many of these alloys can be bonded together by diffusion bonding; that is, the solid-state, metallurgical joining of metal surfaces by applying appropriate temperature and pressure for a time sufficient to permit co-mingling of atoms at the joint interface. In combination, these two techniques promise greater manufacturing efficiency, lower labor costs and great material savings through much reduced machining.
Superplastic forming and diffusion bonding are often accomplished through hot isostatic pressing in which a uniform pressure is applied while the components are maintained at a suitable high temperature.
Hot isostatic pressing of flat or nearly flat parts has long been used to form metal, plastic and composite parts to precise dimensions. Typically, a die having a forming surface is placed with the forming surface uppermost. The material to be formed is placed on the forming surface and a blanket or bag is placed over the assembly. The enclosed space is evacuated or flushed with an inert gas. The assembly is placed in an autoclave and subjected to high temperatures and pressures for an appropriate period. While this process is very effective for producing flat or nearly flat structures, problems are encountered with more three-dimensional structures, especially with hollow structures.
Attempts have been made to design complex, removable molds for hot isostatic pressing of complex or hollow shapes. Typical of these is the mold system disclosed by Borchert et. al. U.S. Pat. No. 4,575,327. These molding systems require a large number of parts, sliding together at angles which will permit removal after molding. The molds are expensive, have a short life, difficult to design, and produce imprecise, out-of-tolerance parts with flash or other surface irregularities unless very carefully assembled.
In some cases, particulate material has been used to apply approximately isostatic pressure for hot isostatic pressing of complex parts. Such an arrangement is described, for example, by Rigby et. al. in U.S. Pat. No. 4,552,710. Precise shaping is difficult with such materials and interaction between particles may prevent true isostatic pressure application.
In some cases, superplastic forming and diffusion bonding are combined in a two step process. For example, as disclosed by Cogan in U.S. Pat. No. 4,071,183, two parts can be formed by superplastic forming, then reinforcing pieces can be placed between the parts and diffusion bonded thereto. This complex method has difficulty in obtaining proper alignment of parts and obtaining uniform diffusion bonding.
Simultaneous superplastic forming and diffusion bonding is possible with simple structures, such as is shown by Elrod in U.S. Pat. No. 4,263,375. Here, a simple rib at the bottom of a rectangular cavity is diffusion bonded to a sheet which is pressed down into the cavity and into contact with the rib by gas pressure. This method is effective with simple structures but cannot accommodate hollow structures or those with significant undercuts.
Thus, there remains an unmet need for a method and apparatus for superplastic forming and diffusion bonding of hollow structures with undercuts or other mold interference areas.